SNAA361A april   2022  – may 2023 LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Reference Frequency
  6. 3Standard LMX2820 Phase Noise Performance
  7. 4 Pre-multiplier Stage Performance
    1. 4.1 External VCO Loop Filter Design
    2. 4.2 Active Loop Filter Implementation
    3. 4.3 External VCO Performance Results
  8. 5External PFD Topology
    1. 5.1 External PFD Loop Filter Modification
    2. 5.2 External PFD Measurement Results
  9. 6Putting it Together in the Real World
    1. 6.1 Real-World Performance Results
    2. 6.2 Why use the Dual Approach?
  10. 7Conclusion
  11. 8Revision History

1 Introduction

The LMX2820 RF synthesizer inherently provides excellent phase noise performance and operates up to 22 GHz; however, there are cases that need improved phase noise performance. The LMX2820 offers a couple of different topologies to improve synthesizer phase noise. One technique bypasses the internal voltage-controlled oscillator (VCO) and uses an external one. This introduces a superior low phase noise VCO while maintaining the programming features of the PLL device. The second technique uses an external mixer to down-convert the RF frequency to the PFD (Phase Frequency Detector) frequency. This bypasses the internal feedback dividers which are one of the sources for degraded PLL phase noise.